JPH05196890A - Optical isolator - Google Patents

Abstract

PURPOSE:To provide the optical isolator which is used suitably for a long- distance, wide-range optical fiber communication system by reduce the polarized wave dispersion of the optical isolator. CONSTITUTION:Two optical isolators which transmit optional forward polarized light are longitudinally connected and the polarizers 604, 606, and 608, and 104, and 106, and 108 of the individual optical isolators are made of birefringent media; and the optical isolators are so arranged that when mutually orthogonal polarized light beams pass through the two cascaded optical isolators forward, the optical path difference between those polarized light beams is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an isolator in the light wave region, and more particularly to an optical isolator suitable for optical fiber communication.

[0002]

2. Description of the Related Art In recent years, with the new development of optical fiber communication, various forms of optical fiber networks have been constructed, and various optical parts have come to be used in these networks. Therefore, in order to maintain the quality of communication, it has become more important than ever before to eliminate the reflected return light from the end face of each optical component. Light that has propagated through an ordinary optical fiber generally has an arbitrary polarization state, and the polarization state changes due to ambient temperature changes, bending and twisting of the fiber, and the like. Therefore, the optical isolator used in the optical fiber communication network needs to have no polarization dependence not only in the backward direction but also in the forward direction incident light. As such a polarization-independent optical isolator, conventional optical isolators disclosed in Japanese Patent Publication No. 60-49297, Japanese Patent Publication No. 61-58809, and 1991 Spring Conference of the Institute of Electronics, Information and Communication Engineers, Vol. Isolators are known.

The structure of the optical isolator disclosed in Japanese Patent Publication No. 61-58809 is shown in FIG. The Faraday rotator 512 for rotating the polarization plane by 45 degrees is composed of two polarizers 511,
It is sandwiched by 513. The polarizer is a wedge-shaped birefringent crystal such as rutile or calcite, and functions to refract only extraordinary rays. Polarizers 511 and 51
The optical axis of 3 is orthogonal to the traveling direction 505 of light, and the optical axis of the second polarizer 513 is the rotating direction of the Faraday rotator 512 around the traveling direction of the light beam with respect to the optical axis of the first polarizer 511. It has rotated 45 degrees. The Faraday rotator 512 is usually installed in a doughnut-shaped permanent magnet in order to apply a magnetic field in the traveling direction of light, but this magnet is omitted in the drawings (this is shown in other drawings of the present application. Is also the same). FIG. 5A shows the path of the forward light rays, and FIG. 5B shows the path of the backward light rays.
Reference numerals 5 and 507 represent ordinary rays, and broken lines 506 and 508 represent extraordinary rays. In the forward light, the polarization of the ordinary ray in the first polarizer 511 becomes the ordinary ray in the second polarizer 513, and the polarization of the extraordinary ray in the first polarizer 511 becomes the extraordinary ray in the second polarizer 513. Therefore, the refraction of the two polarizers of both polarized lights is canceled out, and the emitted lights become 505 and 506 which are parallel to the incident light and are condensed on the fiber 504 by the lens 503. On the other hand, with respect to the light rays in the opposite direction, the polarization of the ordinary ray in the second polarizer 513 becomes an extraordinary ray in the first polarizer 511, and the polarization of the extraordinary ray in the second polarizer 513 becomes an extraordinary ray in the first polarizer 511. Since it becomes an ordinary ray, the refraction of the two polarizers in both polarizations does not cancel out, and the emitted light 507, 50 is not parallel to the incident light.
Therefore, the lens 502 does not focus the light on the fiber 501. Therefore, isolation is realized.

Next, FIG. 6 shows the structure of the optical isolator shown in the 4th volume, 4-125, Volume 4 of the 1991 Spring National Convention of the Institute of Electronics, Information and Communication Engineers. This isolator has a first polarizer 604, a 45-degree rotating Faraday rotator 605, a second polarizer 606, a 45-degree rotating Faraday rotator 607, and a third polarizer 608 in the order in which light travels in the forward direction. Lined up. Each polarizer is made of birefringent crystal such as rutile, whose optic axis is tilted at 45 ° with respect to the traveling direction of light, and shifts only the extraordinary ray light beam in parallel to make the ordinary ray light beam. It has the function of transmitting as it is. Similar to the conventional example of FIG. 5, for the light rays in the forward direction, the first polarizer 604 converts the ordinary polarization 2 into the second and third polarizers 60.
6, 608 also becomes an ordinary ray, and the polarization 3 of the extraordinary ray in the first polarizer 604 becomes an extraordinary ray also in the second and third polarizers 606, 608. On the other hand, with respect to light rays in the opposite direction, the polarization of ordinary rays is generated by the third polarizer 608.
Is an extraordinary ray, and the extraordinary ray is polarized by the third polarizer 608 so that the polarization of the extraordinary ray is an ordinary ray by the second polarizer 606. 3 so that the sum of shift vectors becomes zero
The thickness ratio of one polarizer is 1: √2: 1, and the shift directions are 0 °, −135 °, and 90 ° when the horizontal direction is 0 °. Therefore, in the forward direction, the axis of the incident light to the optical isolator and the axis of the emitted light coincide with each other, but in the reverse direction, the relationship between the polarization in the second polarizer 606 and the crystal optical axis is the same as that of the third polarizer 608. Therefore, the axis of the emitted light remains shifted from the incident light and the isolation can be realized.

That is, in any of the conventional examples, all polarizers (normal ray or extraordinary ray) in the forward direction.
It is devised so that the relationship between the polarized light and the crystal optical axis is the same and that the relationship is reversed in at least one or more polarizers in the opposite direction. In this respect, the optical isolator disclosed in Japanese Patent Laid-Open No. 60-49297 is also the same.

[0006]

In recent years, the development of optical fiber amplifiers and the like has led to the realization of an optical fiber communication system that is longer and more repeatable than before. In such a system, the difference in propagation time between polarizations of light orthogonal to each other, that is, polarization dispersion, is one of the factors that limit the transmission distance and the transmission band.

The optical isolator described in the section of the prior art is
Since a birefringent crystal is used for the polarizer, and the light in the forward direction always has the same polarization and crystal optic axis relationship in all the polarizers, it passes through the optical isolator in the forward direction and is orthogonal to each other. An optical path length difference between the ordinary and extraordinary rays of the polarizer is obtained by multiplying the thickness of the polarizer and the number of polarizers, which causes polarization dispersion and limits the transmission distance and transmission band. To do.

An object of the present invention is to provide an optical isolator which reduces the polarization dispersion of the above optical isolator and is suitable for a long-distance and wide-band optical fiber communication system.

[0009]

The optical isolator of the first structure according to the present invention has a structure in which an even number of optical isolators that transmit forward light of arbitrary polarization are connected in cascade. The optical path length difference between the polarized light is zero when the light passing through the even number of optical isolators, which are made of a medium having birefringence and are orthogonal to each other, pass in the forward direction. It is an optical isolator characterized by arranging each isolator.

An optical isolator having a second structure according to the present invention is an optical isolator which includes a polarizer in an optical path thereof and transmits light of an arbitrary polarization in a forward direction, wherein the polarizer comprises a plurality of birefringent media. A polarizer in which the birefringent media are arranged so that the polarization paths orthogonal to each other pass through the polarizer so that the optical path length difference between the polarized lights becomes a drop when the polarized lights cross each other. Is an optical isolator.

An optical isolator having a third structure according to the present invention is an optical isolator comprising a plurality of polarizers that transmit light of an arbitrary polarization in the forward direction, wherein the polarizer is made of a medium having birefringence, Light of any polarization incident from any direction becomes an ordinary ray at least once in one polarizer,
In addition, the optical isolator is characterized in that the polarizer is arranged so as to become an extraordinary ray in another polarizer at least once.

An optical isolator having a fourth structure according to the present invention is an optical isolator which transmits light of arbitrary polarization in the forward direction, and when polarized lights orthogonal to each other pass in the forward direction, the optical path length between these polarized lights is increased. It is an optical isolator characterized in that a phaser is arranged in the optical path so that the difference becomes zero.

[0013]

As described in the section of the prior art, when the conventional optical isolator is in the forward direction, the polarization of ordinary light and extraordinary light in the first polarizer is the same as that in all other polarizers. Become light. Therefore, an optical path length difference corresponding to the refractive index difference between the ordinary ray and the extraordinary ray always occurs between the polarized lights orthogonal to each other. On the other hand, in the optical isolator according to the present invention, when a certain linearly polarized light passes through the optical isolator in the forward direction, it always takes the polarization state of both the ordinary ray and the extraordinary ray. Therefore, the optical path length difference caused by the difference in the refractive index between the ordinary light and the extraordinary light between the polarizations orthogonal to each other is canceled out, and the optical path length difference can be reduced. Therefore, according to the present invention, it is possible to provide an optical isolator having a small polarization dispersion.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. 1 is an embodiment of an optical isolator according to claim 1 of the present invention. This embodiment has a structure in which two conventional optical isolators shown in FIG. 6 are connected in series in the same direction. Light traveling from left to right in FIG. 1 is a light beam in the forward direction, and the Faraday rotators 605, 607, 105, 10
7 has a polarization plane of 45 in the counterclockwise direction when viewed from the incident plane in the forward direction.
Rotate it once. The left optical isolator in FIG. 1 is called a first optical isolator, and the right optical isolator is called a second optical isolator. The optical axis 109 of the first polarizer 104 of the second optical isolator rotates the Faraday rotator about the optical axis 610 of the third polarizer 608 of the first optical isolator about the traveling direction 1 of the light ray. Two optical isolators are arranged so as to rotate 90 degrees in the direction.
(Originally, the third polarizer 608, 1 of each optical isolator
08 optical axes 610 and 110 are associated with the first polarizer 604 and
Each of the polarizers 10 of the second optical isolator is rotated by 90 degrees in the rotation direction of the Faraday rotator about the traveling direction 1 of the light ray with respect to the optical axes 609 and 109 of 104.
The optical axes of 4, 106 and 108 are rotated by 180 degrees with respect to the optical axes of the corresponding polarizers 604, 606 and 608 of the first optical isolator. ) Therefore, the light ray in the forward direction becomes the extraordinary ray in the polarizer of the first optical isolator, and the polarization 2 which was the ordinary ray in the polarizer of the first optical isolator becomes the extraordinary ray in the polarizer of the second optical isolator. The polarized light 3 which has been described above becomes an ordinary ray in the polarizer of the second optical isolator. Since the thicknesses of the corresponding polarizers of the two optical isolators are the same, the light that has passed through these two optical isolators in the forward direction always has the same optical path length regardless of the polarization, and the polarization dispersion Becomes zero in principle. In this configuration, the amount of shift of the light beam in the opposite direction is √2 times that when one optical isolator is provided, and the isolation is improved. Further, the optical isolator has a feature that the polarization planes 2 and 3 of the incident light are preserved and emitted.

Further, in the structure in which two conventional optical isolators shown in FIG. 5 are connected in series in the same direction, the optical axis of the first polarizer of the second optical isolator is the first optical axis as in the case of FIG. To rotate 90 degrees about the traveling direction of the ray with respect to the optical axis of the second polarizer of the optical isolator.
Even if two optical isolators are arranged, the effect of making the polarization dispersion zero can be obtained.

FIG. 2 shows an embodiment of an optical isolator according to claim 2 of the present invention. This optical isolator comprises three polarizers 604, 60 of the conventional optical isolator shown in FIG.
6 and 608, new polarizers 204, 206, and 208 having the same crystal and the same thickness are added, and the optical axes of the new polarizers travel with respect to the optical axes of the original polarizers. It is installed so as to rotate 90 degrees in the same direction with the direction 1 as the axis. Therefore, the original polarizers 604, 60
Polarization 2 which is an ordinary ray at 6,608 becomes an extraordinary ray at the newly added polarizers 204, 206 and 208, and polarization 3 which is an extraordinary ray at the original polarizer becomes an ordinary ray at the newly added polarizer. Therefore, the light passing through the pair of the original polarizer and the new polarizer always has the same optical path length regardless of the polarization, and the polarization dispersion is theoretically zero. Since the Faraday rotators 205 and 207 do not have birefringence, if there is no polarization dispersion in each polarizer set, the polarization dispersion will be zero in the entire optical isolator.

FIG. 3 shows an embodiment of an optical isolator according to claim 3 of the present invention. This optical isolator comprises three polarizers 604, 60 of the conventional optical isolator shown in FIG.
6, 608, the optical axis of the second polarizer 604 is the Faraday rotator 605, 607 with the traveling direction 1 of the light ray as the axis.
The optical axis of the third polarizer 608 is rotated by 180 degrees about the traveling direction of the light beam. Therefore, the forward polarized light 2 which is an ordinary ray in the first polarizer 604 becomes an extraordinary ray in the second polarizer 306 and becomes an ordinary ray in the third polarizer 308. On the contrary, the forward polarization 3 which is an extraordinary ray in the first polarizer 604 becomes an ordinary ray in the second polarizer 306 and becomes an extraordinary ray in the third polarizer 308. As described above, the first, second and third polarizers 60
Since the thickness ratio of 4,306 and 308 is 1: √2: 1, this optical isolator cannot make polarization dispersion zero in principle, but compared with the conventional optical isolator of FIG. Polarization dispersion is (2-√2) / (2 + √2) times (about 17%)
Can be reduced to Further, the present optical isolator has the feature that it can be made thinner than the optical isolators of the first and second aspects.

FIG. 4 shows an embodiment of an optical isolator according to claim 4 of the present invention. This optical isolator is the same as the conventional optical isolator shown in FIG.
08 has a structure in which a retarder 401 having the same crystal as that of 08 and a thickness equal to the sum of the thicknesses of the respective polarizers is added after the third polarizer 608, and the optical axis 402 of the retarder 401 is the traveling direction of light. The optical axis 61 of the third polarizer 608 is orthogonal to the direction 1 and
It is arranged so that 0 is also orthogonal to the axis projected on the plane orthogonal to the traveling direction of light. With this arrangement, for light passing through the optical isolator in the forward direction, the polarization 2 of the ordinary ray in each polarizer becomes an extraordinary ray in the retarder 401, and the polarization 3 of the extraordinary ray in each polarizer becomes the retarder. It becomes an ordinary ray at 401. Therefore, light that has passed through the optical isolator in the forward direction always has the same optical path length regardless of its polarization, and the polarization dispersion is theoretically zero. As long as the optical axis of the retarder has the above-mentioned relationship with the optical axes of the adjacent polarizers, the retarder has no influence on the optical isolation operation. Therefore, the phase shifter 40
Even if 1 is placed in front of the first polarizer 604 or in a place such as between any polarizer and the Faraday rotator, as long as the optical axis of the retarder has the above-described relationship with the optical axis of the adjacent polarizer, The same effect can be obtained. Even if the retarder is divided into several parts, the sum of the thicknesses of the retarders is equal to the sum of the thicknesses of the polarizers, and the optical axis of each retarder is the optical axis of the adjacent polarizer. As long as there is the above relationship, the same effect can be obtained. Even if a crystal different from the polarizer is used as the retarder, the same effect can be obtained by changing the thickness.

[0019]

As described above, according to the present invention, it is possible to provide a polarization-independent optical isolator having no polarization dispersion.

[Brief description of drawings]

FIG. 1 is a perspective view of an embodiment of an optical isolator according to claim 1 of the present invention.

FIG. 2 is a perspective view of an embodiment of an optical isolator according to claim 2 of the present invention.

FIG. 3 is a perspective view of an embodiment of an optical isolator according to claim 3 of the present invention.

FIG. 4 is a perspective view of an embodiment of the optical isolator according to claim 4 of the present invention.

FIG. 5 is a plan view of an example of a conventional optical isolator.

FIG. 6 is a perspective view of an embodiment of an optical isolator according to a conventional example.

[Explanation of symbols]

 1 Forward ray 2 Polarization that becomes an ordinary ray in the first polarizer 3 Polarization that becomes an extraordinary ray in the first polarizer 104 Polarizer 105 Faraday rotator 106 Polarizer 107 Faraday rotator 108 Polarizer 109 Optical axis of polarizer 110 Optical axis of polarizer 204 Polarizer 205 Faraday rotator 206 Polarizer 207 Faraday rotator 208 Polarizer 209 Optical axis of polarizer 210 Optical axis of polarizer 306 Polarizer 308 Polarizer 310 Optical axis of polarizer 401 Phaser 402 Optical axis of phaser 501 Optical fiber 502 Lens 503 Lens 504 Optical fiber 505 Normal ray in the forward direction 506 Extraordinary ray in the forward direction 507 Ordinary ray in the reverse direction 508 Extraordinary ray in the reverse direction 511 Polarizer 512 Faraday rotator 513 Polarizer 604 Polarizer 605 Faraday rotator 60 6 Polarizer 607 Faraday rotator 608 Polarizer 609 Optical axis of polarizer 610 Optical axis of polarizer

Claims (4)

[Claims] 1. A structure in which an even number of optical isolators that transmit light of an arbitrary polarized light in the forward direction are connected in cascade, and the polarizers of the individual optical isolators are made of a medium having birefringence, and polarized lights orthogonal to each other are An optical isolator, wherein each of the optical isolators is arranged such that the optical path length difference between the polarized lights becomes zero when the light passes through the even number of cascaded optical isolators in the forward direction. 2. An optical isolator that includes a polarizer in an optical path and transmits light of an arbitrary polarization in a forward direction, wherein the polarizer is formed by connecting a plurality of birefringent media in cascade, and is orthogonal to each other. An optical isolator, wherein the birefringent medium is arranged so that the optical path length difference between the polarized lights becomes zero when the polarized lights pass through the polarizer. 3. An optical isolator comprising a plurality of polarizers which transmit light of arbitrary polarization in the forward direction, wherein the polarizer is made of a medium having birefringence, and light of arbitrary polarization incident in the forward direction is An optical isolator, wherein the polarizer is arranged so that it becomes an ordinary ray at least once in one polarizer and becomes an extraordinary ray at least once in another polarizer. 4. An optical isolator for transmitting light of arbitrary polarization in the forward direction is provided with a phase shifter so that, when polarized lights orthogonal to each other pass in the forward direction, the optical path length difference between these polarized lights becomes zero. An optical isolator characterized by being placed in the optical path.